In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto an imaging receiving member. Ink jet printers have printheads that operate a plurality of inkjets that eject liquid ink onto the image receiving member. The ink can be stored in reservoirs located within cartridges installed in the printer. Different types of ink can be used in inkjet printers. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers can use ink that is supplied in a gel form. The gel is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the image receiving member. These inks are called phase change inks. Phase change inks remain in a solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The printhead unit ejects molten ink supplied to the unit onto the image receiving member. Once the ejected ink is on image receiving member, the ink droplets solidify. In these solid ink printers, the solid ink can be in the form of pellets, ink sticks, granules or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like.
An inkjet printer can include one or more printheads. Each printhead contains an array of individual nozzles for ejecting drops of ink across an open gap to the image receiving member to form an image. The image receiving member can be a continuous web of recording media, one or more media sheets, or a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media, either continuous or sheet, by a mechanical force in a transfix nip formed by the rotating surface and a transfix roller.
In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, or voltage level, of the signals affects the amount of ink ejected in each drop. The firing signal is generated by a printhead controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
The environment in which printers, printer ink, and image receiving members are used is not always ideal. Several sources of printing errors exist and can result from ink contamination, improper heating of phase change ink, and improper maintenance of a printer. Additionally, not all inkjet nozzles in a printhead remain operational without maintenance. Some inkjet nozzles can become intermittent, meaning the inkjet nozzle can fire some times and not at other times. To reduce or eliminate intermittent firing, ink jet printheads and the reservoirs supplying ink to the nozzles can include filters designed to filter out or block contaminants from entering the inkjets. Other inkjet printers, particularly those depositing phase change ink, include a purge operation where the printhead nozzles are purged of ink on a routine basis.
When a phase change printer is not operated for a period of time, such as overnight, the phase change ink can become viscous or even solidify. This change in state is typically temporary and does not pose a risk to the proper operation of the printer, once the printer has been returned to an operating temperature needed for printing after the period of nonuse. To ensure the printer is ready for printing, a purge operation can be performed to purge the printhead nozzles of any blockages or air bubbles. In some cases, however, the phase change ink can migrate to other locations in the printer, including the printheads, the ink reservoirs, and even ink conduits, where the phase change ink is not sufficiently liquefied due to location. Consequently it is desirable to reduce the likelihood that phase change ink migrates to a location within a printer where proper liquefaction of the phase change ink is difficult, impossible or not economically advantageous.